Ultrafast laser oscillators, such as a Kerr-lens mode-locked, titanium-doped sapphire (Ti:sapphire) laser, can generate a series of short optical pulses. The temporal pulse-length of the output is determined by the laser system dynamics and can vary from less than 10 femtoseconds to more than 1 picosecond. The temporal pulse-length is inversely proportional to the pulse-wavelength and bandwidth. Shorter pulses have wider bandwidths. For example, a 100 fs pulse having a center wavelength at 1000 nm requires at least 11 nm of bandwidth. The temporal-separation and repetition-rate of the pulses is determined by the round-trip time that light takes to oscillate in the laser cavity. A typical ultrafast laser system can operate at a pulse-separation of 10 nanoseconds (ns), i.e., a pulse repetition-rate of 100 Megahertz (MHz). The average power of the laser can vary, from a few milliwatts (mw) to several Watts (W), resulting in pulses having energies from tens of nanojoules (nJ) per pulse, and peak intensities greater than 100 Kilowatts (KW). Ultrafast amplifier systems can increase the energy per pulse to over 1 Joule (J) and average intensities over 1.0 Terawatts (TW). Optical-fibers are used in optical delivery systems for ultrashort pulses because they are versatile and provide good spatial beam quality. For light having a wavelength of about 800 nm, single-mode fibers having very small core diameters, for example, about 5 micrometers (.mu.m) are available. Larger diameter optical-fibers can carry beams with higher order spatial distortions. Single-mode polarization-preserving optical-fibers with built-in birefringence are also available. Optical-fibers are often used to aid delivery of a laser system's optical output to small hand-held devices (handpieces) for laser-surgery or laser material-processing.
Ultrafast (ultrashort) laser-pulses experience linear optical dispersion effects when passing through an optical material, such as the quartz-glass (fused silica) used to form the core of an optical-fiber. Second-order and third-order linear dispersion effects delay the short-wavelength end of the light spectrum of the pulse relative to the long-wavelength end, thus temporally expanding the laser-pulse. The amount of linear dispersion on a given laser-pulse depends upon the bandwidth of the laser-pulse, the dispersion properties of the material, and the length of material traversed by the laser-pulse.
Ultrafast laser-pulses also experience nonlinear optical distortion effects when passing through materials at high power-densities. The nonlinear distortion depends upon the intensity of the laser-pulse, the nonlinear coefficient of the material, and the length of material traversed by the laser-pulse. Nonlinear optical distortion places a maximum limit on the power that can be delivered by an optical-fiber without inducing significant nonlinear distortion of the ultrafast pulse.
Both linear and nonlinear distortions are pronounced when using single-mode optical-fibers because of the very long optical path lengths (typically several meters); high power-densities due to the very small beam size (core diameter of the fiber, typically about 5 .mu.m) in the fiber; and the short temporal pulse width. For these reasons, using optical-fibers to deliver ultrafast laser system output has not been widespread in applications that need to preserve the temporal character of the laser-pulses.
One proposed solution to power handling problems of above-described problems is disclosed in U.S. Pat. No. 4,918,751 granted to Pessot et al. Here, a laser-pulse is temporally-expanded (stretched) by a pair of diffraction-gratings before being injected into an optical-fiber. The laser-pulse is temporally-expanded to an extent that recompression effects of the optical-fiber are insufficient to raise the pulse intensity in the optical-fiber to a point at which nonlinear effects become significant. After exiting the optical-fiber, the laser-pulse is further compressed, using a second diffraction-grating-pair, to a desired pulse-length.
There are several drawbacks to the solution proposed by Pessot et al. By way of example, optical-fibers induce a negative second-order linear dispersion and a positive third-order linear dispersion on light passing therethrough. Diffraction-gratings of the type used to broaden the pulse before it enters the fiber introduce a negative second-order dispersion but a positive third-order dispersion. Because of this, while the diffraction-gratings are counteracting the second-order linear dispersion of the fiber, they compound the third-order linear dispersion.
Further by way of example, the pulse communication system of Pessot et al. is conceived for fixed transmitter and receiver arrangements for optical communication systems. In such systems, there are no particular constraints on space and complexity of the pulse-compression and pulse-expansion scheme. Uses for ultrafast laser-pulses, however, now are rapidly expanding into areas such as scientific and industrial instruments, and laser medical and dental applications wherein a compact or portable device is generally preferable to a bulky device.
In medical and dental applications in particular, laser-light for therapeutic or surgical uses is often delivered to a treatment site by an optical-fiber which terminates, at the delivery-end thereof, in a handpiece used by a doctor performing the treatment. Complexity aside, the space required for a diffraction-grating pulse-compressor would make it impractical for incorporation in such a handpiece.
There is a need for a delivery system that can deliver ultrafast laserpulses using an optical-fiber in a manner that allows the laser-pulses to traverse the delivery system without causing significant nonlinear distortion of the laser-pulses and fully compensates for both second and third-order linear dispersion effects in the optical-fiber. The system should preferably not require a complex, diffraction-grating pulse-compressor at the delivery end of the optical-fiber.